[llvm-bugs] [Bug 38327] New: Subtract stack pointer instead of push

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38327

            Bug ID: 38327
           Summary: Subtract stack pointer instead of push
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

Code:

int is_enabled(void);


int tobool(void) {
    return !!is_enabled();
}

int tobool2(void) {
    if(is_enabled())
        return 9;
    return 4;
}

Clang -O3:
tobool:                                 # @tobool
        push    rax
        call    is_enabled
        xor     ecx, ecx
        test    eax, eax
        setne   cl
        mov     eax, ecx
        pop     rcx
        ret
tobool2:                                # @tobool2
        push    rax
        call    is_enabled
        xor     ecx, ecx
        test    eax, eax
        setne   cl
        lea     eax, [rcx + 4*rcx]
        add     eax, 4
        pop     rcx
        ret

GCC avoid push, it uses sub rsp, 8.
tobool:
        sub     rsp, 8
        call    is_enabled
        test    eax, eax
        setne   al
        add     rsp, 8
        movzx   eax, al
        ret
tobool2:
        sub     rsp, 8
        call    is_enabled
        cmp     eax, 1
        sbb     eax, eax
        add     rsp, 8
        and     eax, -5
        add     eax, 9
        ret


Maybe it could be worth (less instructions, register pressure) to follow GCC
decision, I think.

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